Magnetic recording medium

ABSTRACT

The present invention provides a magnetic recording medium having high electromagnetic transfer characteristics in a high-density recording medium. The invention provides a magnetic recording medium, which comprises a nonmagnetic primer layer placed on a nonmagnetic support member and at least one layer of magnetic layers with ferromagnetic powder dispersed in a binder, said magnetic layer being placed on said primer layer, whereby diamond particles with average particle size of 0.05 to 1 μm are added to the magnetic layer, and at least one type of compound selected from the following genera formulae (1) and (2) is contained:                    
     where R 1  represents an alkyl group having 1 to 2 carbon atoms; and 
     R 2 , R 3  and R 4  each represents a hydrocarbon group having 4 to 21 carbon atoms.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic recording medium forhigh-density recording containing ferromagnetic fine powder as amagnetic layer. In particular, the invention relates to a magneticrecording medium having good storage property and high durability.

As magnetic recording medium for the applications in audio equipment,video equipment, computer, etc., a magnetic recording medium is used,which has a magnetic layer containing ferromagnetic powder dispersed ina binder, and the magnetic layer is placed on a nonmagnetic supportmember.

In recent years, digital recording with less deterioration of recordingquality compared with conventional analog recording has been used widelyin practical application, also in the field of video tape recorder forhome use. In general, more signals must be recorded in digital recordingthan in analog recording. The recording and reproducing system andrecording medium used for digital recording must provide high picturequality and high tone quality and also must be designed in more compactsize and must have space-saving property. In this respect, there arestrong demands on high-density recording.

To achieve high-density recording, it is necessary to turn the recordingsignals to shorter wavelength and recording track must be designednarrower. For this purpose, ferromagnetic powder must be finer powderwith high filling ratio and the recording medium must have smoothersurface. Also, writing speed and reading speed to and from the recordingmedium must be increased. Attempts are now being made to increase thenumber of revolutions of cylinder or to increase carrier speed of tape.

In the equipment or devices using the magnetic recording medium, thereis problem in that magnetic head is contaminated because the medium andthe magnetic head slide against each other. In particular, in the devicefor high density recording, number of revolutions of the magnetic headis high. In a digital video tape recorder, number of revolutions of themagnetic head is as high as 9,600 rpm, and this is much higher comparedwith 1,800 rpm of an analog video tape recorder for household use, and5,000 rpm of a video tape recorder for business use.

With the increase of sliding speed between the magnetic recording mediumand the magnetic head, there are now strong demands on the developmentof a magnetic recording medium, which has high durability and highresistance to damage and has high resistance to high speed slidingmovement.

Not only for the tape-type magnetic recording medium, but also fordisk-type magnetic recording medium, high-density magnetic recordingmedium is required, as typically represented by Zip (Iomega Inc.), whichcan be rotated at higher speed compared with the conventional typefloppy disk. Thus, a magnetic recording medium with high durability andhigh resistance to wear and damage is required for this purpose.

To solve the above problems, for the purpose of providing a magneticrecording medium, which contains ferromagnetic metal powder dispersed ina binder and which has high durability in high density recording and canperform stabilized recording and reproduction, it is proposed to use amagnetic recording medium, which contains various types of lubricants inthe magnetic layer. It is proposed to use various types of triester ortetraester compound as lubricant when ester is used as lubricant.

For instance, JP-88021255(B) describes the use of triester or tetraesterlubricant obtained from trimethylolpropane, trimethylolethane orpentaerythritol. However, these lubricants have poor storage property,and the resistance to damage of the magnetic layer is low. Inparticular, the properties of this lubricant are not high enough forhigh-density recording medium such as digital recording tape.

Further, JP-B-61-26134 describes a magnetic recording medium usingtriester of trimethylolpropane as lubricant. However, it has poorstorage property and low resistance to damage. Also, electromagnetictransfer characteristics are not high enough as a high-density magneticrecording medium.

Also, JP-59065931(A) describes a magnetic recording medium using alubricant, which simultaneously uses triester of trimethylolpropane andother diester or tetraester and monoester. However, storage property ofthe magnetic layer is not sufficiently high, and the magnetic layer alsohas poor resistance to damage or wear. Further, these products are lowin durability and have poor electromagnetic transfer characteristics.

JP-61139921(A) describes a magnetic recording medium, which uses fattyacid ester of polyhydric alcohol and phosphoric acid ester ofphenoxydiethylene glycol as lubricants. But, this product has magneticlayer with low storage property and low resistance to damage. Further,the product has low durability and poor electromagnetic transfercharacteristics.

Further, U.S. Pat. No. 4,696,869 (JP-95015748(B)) describes a magneticrecording medium, using ester or trimethylolpropane or ester ofpentaerythritol and monoester as lubricants. However, the magnetic layerhas low storage property and low resistance to damage. Further, thisproduct has low durability and poor electromagnetic transfercharacteristics in high-density recording.

Also, JP-2552958 proposes a magnetic recording medium, which can havehigher electromagnetic transfer characteristics in short wavelengthrecording and which comprises a primer layer and a thin upper magneticlayer. In the prescription as disclosed, the product is not good enoughin terms of durability.

JP-A-06052541(A) proposes a magnetic recording medium, which comprises amagnetic layer containing abrasive material with Morse hardness of 8 ormore, whereby average height of projections of the abrasive material is15 nm or less. However, this is not satisfactory to provide highdurability.

Further, U.S. Pat. No. 6,096,406 (JP-11086273(A)) proposes a magneticrecording medium, which comprises a substantially nonmagnetic lowerlayer placed on a support member, and a magnetic layer containingferromagnetic powder dispersed in a binder being placed thereon, wherebycoercive force is set to 1800 Oe or more, and the magnetic layercontains diamond particles having average particle size of 0.1 to 1.0μm. However, this is not satisfactory to provide high durability.

It is an object of the present invention to provide a magnetic recordingmedium having high durability. The invention provides a magneticrecording medium having high electromagnetic transfer characteristics inhigh-density recording. Further, the invention provides a magneticrecording medium having high calender moldability and superb surfacesmoothness.

SUMMARY OF THE INVENTION

The present invention provides a magnetic recording medium, whichcomprises a nonmagnetic primer layer placed on a nonmagnetic supportmember and at least one layer of magnetic layers with ferromagneticpowder dispersed in a binder, said magnetic layer being placed on saidprimer layer, whereby at least one type of compound selected from thefollowing genera formulae (1) and (2) is contained:

where R¹ represents an alkyl group having 1 to 2 carbon atoms; and

R², R³ and R⁴ each represents a hydrocarbon group having 4 to 21 carbonatoms.

Also, the present invention provides the magnetic recording medium asdescribed above, wherein thickness of the magnetic layer is within therange of 0.05 to 1 μm.

Further, the present invention provides the magnetic recording medium asdescribed above, wherein said medium is a disk-type magnetic recordingmedium.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a high-density magnetic recording medium,comprising a lubricant of specific chemical structure for the purpose ofachieving high durability and high resistance to damage and goodelectromagnetic transfer characteristics.

In particular, it has been found in the present invention that, when atetraester compound of the present invention is added at least to aprimer layer which contains a nonmagnetic primer layer, and a magneticlayer is coated on the primer layer and dried, and is further processedby calender processing, very smooth magnetic layer can be obtained.Thus, a magnetic recording medium having high durability under hightemperature and high speed operation can be obtained.

As the lubricant to be used in the present invention, it is preferableto use a tetraester compound expressed by the following general formula:

where R¹ represents an alkyl group having 1 to 2 carbon atoms; and

R², R³ and R⁴ each represents a hydrocarbon group having 4 to 21 carbonatoms.

In the magnetic recording medium of the present invention, it ispreferable that the medium contains at least a compound expressed by thegeneral formulae (1) or (2).

Preferably, R¹ represents a methyl group or an ethyl group.

Also, R², R³ and R⁴ each preferably represents a hydrocarbon grouphaving 8 to 17 carbon atoms. R², R³ and R⁴ may be the same or differentfrom each other. In R², R³ and R⁴, if the number of carbon atoms in thehydrocarbon group is 4 or less, the product is too volatile. When themagnetic recording medium is turned to high temperature under friction,the quantity of the lubricant in the surface of the magnetic layer isdecreased, and this results in lower durability. On the other hand, ifthe number of carbon atoms is increased, viscosity increases, and liquidlubricating property is decreased. As a result, durability is decreased.

As the hydrocarbon group, saturated hydrocarbon group or unsaturatedhydrocarbon group may be used. In general, it is preferable to usesaturated hydrocarbon group from the viewpoints of storage andstability. Also, branched or direct-chain hydrocarbon group may be used,while direct-chain group is preferably used because a magnetic recordingmedium with low viscosity and high durability can be obtained.

As the triester compound expressed by the general formula (1) or thegeneral formula (2), the following compounds may be used:

It is preferable that the magnetic recording medium of the presentinvention contains 1 to 20 weight parts of the triester compound to 100weight parts of the nonmagnetic powder. More preferably, it is in therange of 1 to 13 weight parts.

In the magnetic recording medium of the present invention, in additionto the lubricant comprising tetraester as expressed by the generalformulae (1) or (2), an additive having lubricating effect, anti-staticeffect, dispersing effect, plasticizing effect, etc. may be used. Forexample, molybdenum disulfide, tungsten disulfide, graphite, boronnitride, graphite fluoride, silicone oil, silicone having polar group,fatty acid denatured silicone, fluorine-containing silicone,fluorine-containing alcohol, fluorine-containing ester, polyolefin,polyglycol, alkyl phosphoric acid ester and its alkali metal salt, alkylsulfuric acid ester and its alkali metal salt, polyphenylether,phenylphosphonic acid, aminoquinones, various types of silane couplingagents, titanium coupling agents, fluorine-containing alkyl sulfuricacid ester and its alkali metal salt, monobasic fatty acid having 10 to24 carbon atoms (may contain unsaturated bonding or may be branched) andmetal salt (such as Li, Na, K, Cu, etc.), or monohydric, dihydric,trihydric, tetrahydric, pentahydric, or hexahydric alcohol having 12 to22 carbon atoms (may contain unsaturated bonding or may be branched),alkoxy alcohol having 12 to 22 carbon atoms, mono-fatty acid ester ordi-fatty acid ester comprising either one of monobasic fatty acid having10 to 24 carbon atoms (may contain unsaturated bonding or may bebranched) or monohydric, dihydric, trihydric, tetrahydric, pentahydricor hexahydric alcohol having 2 to 12 carbon atoms (may containunsaturated bonding or may be branched), or fatty acid ester ofmonoalkylether of alkylene oxide polymerized product, fatty acid amidehaving 8 to 22 carbon atoms, aliphatic amine having 8 to 22 carbonatoms, etc. may be used.

As the monoester compound, it is preferable to use saturated fatty acidmonoester, unsaturated fatty acid monoester, ester of alkylene oxideadded alcohol and fatty acid, etc.

Also, it is preferable to use n-butyl stearate, sec-butyl stearate,n-butyl palmitate, n-butyl myristate, isoamyl stearate, isoamylpalmitate, isoamyl myristate, 2-ethylhexyl stearate, 2-ethylhexylpalmitate, 2-ethylhexyl myristate, oleyl oleate, oleyl stearate, stearylstearate, butoxyethyl stearate, butoxydiethylene glycol stearate, etc.

As the fatty acid, it is preferable to use palmitoleic acid, oleic acid,erucic acid, linoleic acid, stearic acid, palmitic acid, myristic acid,etc.

As the binder suitable for the magnetic layer and the primer layer,thermoplastic resin, thermosetting resin, reactive resin or mixture ofthese compounds may be used. As the thermoplastic resin, it ispreferable to use the resin, which has glass transition temperature of−100° C. to +150° C., number average molecular weight of 1,000 to200,000, or more preferably 10,000 to 100,000, and degree ofpolymerization of about 50 to 1,000 may be used.

As these compounds, polymer or copolymer containing the followingsubstance as constituent units or polyurethane resin, or various typesof rubber type resin may be used: vinyl chloride, vinyl acetate, vinylalcohol, maleic acid, acrylic acid, acrylic acid ester, vinylidenechloride, acrylonitrile, methacrylic acid, methacrylic acid ester,styrene, butadiene, ethylene, vinylbutyral, vinylacetal, vinylester,etc. As the thermosetting resin or reactive resin, phenol resin, epoxyresin, polyurethane curing resin, urea resin, melamine resin, alkydresin, acryl type reactive resin, formaldehyde resin, silicone resin,epoxy-polyamide resin, mixture of polyester resin and isocyanateprepolymer, mixture of polyester polyol and polyisocyanate, mixture ofpolyurethane and polyisocyanate, etc. may be used. For further detailson these resin compounds, reference should be made to: “Handbook ofPlastics” published by Asakura Shoten Co., Ltd. Electron beam curingresin already known in the art may be used in each of the above layers.The examples and the manufacturing method are described in detail inJP-A-62-256219. The above resin compounds may be used alone or incombination. As the preferable combinations, a combination ofpolyurethane resin with at least one type selected from vinyl chlorideresin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylacetate-vinyl alcohol copolymer, or vinyl chloride-vinyl acetate-maleicacid anhydride copolymer, or a combination of polyisocyanate with thesecompounds may be used.

The tetraester compound of the present invention has high affinity tovinyl chloride type binder or polyurethane type binder, and it ispreferable to use these compounds as the binder. In particular, as thebinder used in the primer layer, it is preferable to use vinyl chloridetype binder or polyurethane type binder.

The vinyl chloride type binder may be copolymerized with the followingcompounds: acrylic or methacrylic monomer such as alkyl acrylate, alkylmethacrylate, etc., allyl ether such as allylalkylether, fatty acidvinyl ester such as vinyl acetate, vinyl propionate, etc., vinyl monomersuch as styrene, ethylene, butadiene, etc., and further, monomer havingfunctional groups such as hydroxyl group, epoxy group, etc. or polargroup as to be described later.

As the polyurethane, polyester urethane, polyether urethane,polyetherester urethane, acrylic polyurethane, etc. may be used.

The polyurethane having glass transition temperature (Tg) of −50° C. to+200° C. is preferably used, or more preferably 20° C. to 100° C. Ifglass transition temperature is too low, durability is decreased. If itis too high, calender moldability is decreased, and this leads to poorsmoothness and low electromagnetic transfer characteristics.

In the binder, it is preferable that —COOM, —SO₃M, —SO₄M, —PO(OM)₂,—OPO(OM)₂, amino group, quaternary ammonium base, etc. are introduced aspolar groups in an amount of 1×10⁻⁵ eq/g to 2×10⁻⁴ eq/g. If the amountof these polar groups is lower than 1×10⁻⁵ eq/g, dispersion property isdecreased. If it is higher than 2×10⁻⁴ eq/g, dispersion property is alsodecreased.

It is preferable that OH group is introduced as curing functional groupwith isocyanate curing agent, or epoxy group, SH group, CN group, —NO₂group, etc. may be introduced.

It is preferable that the binder including curing agent is contained inthe magnetic layer in an amount of 10 to 25 weight parts to 100 weightparts of the ferromagnetic powder.

As concrete examples of the binders to be used in the present invention,the following products may be used: VAGH, VYHH, VMCH, VAGF, VAGD, VROH,VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, and PKFE (manufacturedby Union Carbide Corporation), MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN,MPR-TMF, MPR-TS, MPR-TM, and MPR-TAO (manufactured by Nisshin ChemicalIndustry Co., Ltd.), 1000W and 100FD (manufactured by Denki KagakuK.K.), MR-104, MR-105, MR110, MR100, MR555, and 400X-110A (manufacturedby Nippon Zeon Co., Ltd.), Nipporan N2301, N2302, and N2304(manufactured by Nippon Polyurethane Co., Ltd.), Pandex T-5105, T-R3080,T-5201, Barnock D-400, D-210-80, Crisbon 6109, and 7209 (manufactured byDainippon Ink & Chemicals, Inc.), Vylon UR8200, UR8300, UR-8700, RV530,and RV280 (manufactured by Toyobo Co., Ltd.), Daiphelamine 4020, 5020,5100, 5300, 9020, 9022, and 7020 (manufactured by Dainichi Seika Co.,Ltd.), MX5004 (manufactured by Mitsubishi Chemical Corporation),Sanprene SP-150 (manufactured by Sanyo Kasei Co., Ltd.), and Saran F310and F210 (manufactured by Asahi Chemical Industry Co., Ltd.).

As the ferromagnetic powder to be used in the magnetic layer of thepresent invention, it is preferable to use ferromagnetic alloy powdercontaining α—Fe as main component. The ferromagnetic powder may containthe following elements in addition to the atoms as already designated:Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta,W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, etc. Inparticular, it is preferable that at least one of Al, Si, Ca, Y, Ba, La,Nd, Co, Ni or B is contained in addition to α—Fe, or more preferably atleast one of Co, Y or Al. The content of Co is preferably 0 to 40 atom %inclusive to α—Fe, or more preferably 15 to 35 atom % inclusive, or mostpreferably 20 to 35 atom % inclusive. The content of Y is preferably 1.5to 12 atom % inclusive to α—Fe, or more preferably 3 to 10 atom %inclusive, or most preferably 4 to 9 atom % inclusive. The content of Alis preferably 5 to 30 atom % inclusive to α—Fe, or more preferably 5 to15 atom % inclusive, or most preferably 7 to 12 atom % inclusive. Theferromagnetic powder may be processed in advance with dispersing agent,lubricant, surface active agent, anti-static agent, etc. beforedispersion. More concretely, the details are described in:JP-69014090(B), JP-70018372(B), JP-72022062(B), JP-72022513(B),JP-71028466(B), JP-71038755(B), JP-72004286(B), JP-72012422(B),JP-72017284(B), JP-72018509(B), JP-7218573(B), JP-64010307(B),JP-71039639(B), U.S. Pat. No. 3,026,215, U.S. Pat. No. 3,031,341, U.S.Pat. No. 3,100,194, U.S. Pat. No. 3,242,005, US-3,389,014, etc.

A small quantity of hydroxides or oxides may be contained in theferromagnetic alloy powder. The ferromagnetic alloy powder obtained bythe manufacturing methods already known may be used in the presentinvention, and the following methods may be used: A method to reducecomplex organic acid salt (mainly oxalic acid salt) using the reducinggas such as hydrogen, a method to obtain Fe or Fe—Co particles byreducing ion oxide with reducing gas such as hydrogen, a method tothermally decompose metal carbonyl compound, a method to reduce byadding reducing agent such as sodium borohydride, hypophosphite orhydrazine to aqueous solution of ferromagnetic metal, a method to obtainfine powder by evaporating metal in an atmosphere of inert gas under lowpressure, etc. The ferromagnetic alloy powder thus obtained may beprocessed by one of the following methods: a method for gradualoxidation, i.e. a method to dry after immersing in organic solvent, amethod to immerse in organic solvent and to form oxide film on thesurface by oxygen-containing gas and to dry, or a method to form oxidefilm on the surface by adjusting partial pressure of oxygen gas andinert gas without using organic solvent.

If the ferromagnetic powder in the magnetic layer of the presentinvention is expressed by specific surface area according to BET method,it is 45 to 80 m²/g, or more preferably 50 to 70 m²/g. If it is lowerthan 40 m²/g, noise increases. If it is 80 m²/g or more, good surfaceproperty is not obtained and this is not desirable. Crystallite size ofthe ferromagnetic powder of the magnetic layer of the present inventionis 8 to 35 nm, or more preferably 10 to 25 nm, or most preferably 14 to20 nm. Longer axis diameter of the ferromagnetic powder is 0.02 to 0.25μm inclusive, or more preferably 0.05 to 0.15 μm inclusive, or mostpreferably 0.06 to 0.1 μm inclusive. Acicular ratio of the ferromagneticpowder is preferably 3 to 15 inclusive, or more preferably 5 to 12inclusive. The value of σs of the magnetic metal powder is preferably100 to 180 Am²/kg (emu/g), or more preferably 110 to 170 Am²/kg (emu/g),or most preferably 125 to 160 Am²/kg (emu/g). Coercive force of metalpowder is preferably 111.4 kA/m to 278.5 kA/m inclusive, or morepreferably 143.3 kA/m to 238.7 kA/m inclusive.

It is preferable that moisture content of the ferromagnetic powder is inthe range of 0.01% to 2%. It is preferable to optimize the moisturecontent of the ferromagnetic powder depending on the type of the binder.It is preferable to adjust pH value of the ferromagnetic powder byadjusting combination with the binder used. The preferable pH range is 4to 12, or it is more preferably 6 to 10. The ferromagnetic powder may beprocessed by surface treatment using Al, Si, P or oxide of theseelements. The amount of the processed part is preferably 0.1 to 10% oftotal ferromagnetic powder. When it is processed by surface treatment,adsorption of lubricant by fatty acid or the like is reduced to 100mg/M² or lower, and this is desirable. The ferromagnetic powder maycontain soluble inorganic ions such as Na, Ca, Fe, Ni, Sr, etc. It ispreferable that these are not present, but even when these are present,if it is in concentration of less than 200 ppm, the properties of theproduct are not particularly affected. It is preferable that theferromagnetic powder used in the present invention has less voids. Thepercentage of the voids contained is preferably 20 vol % or less, ormore preferably 5 vol % or less. The shape of the ferromagnetic powdermay be any of needle-like, grain-like or spindle-like shape so far as itsatisfies the properties for the particle size. Inverted magnetic fielddistribution (SFD) of the ferromagnetic powder itself is preferablylower, i.e. 0.8 or lower. The distribution of the value of Hc of theferromagnetic powder must be lower. If SFD is 0.8 or lower, the producthas good electromagnetic transfer characteristics and higher output.Magnetization inversion is sharp, and peak shift occurs less frequently,and the product is suitable for high-density digital magnetic recording.To decrease distribution of Hc, there are methods such as a method toincrease particle size distribution of goethite in the ferromagneticpowder or a method to prevent sintering.

As the ferromagnetic powder used in the magnetic layer of the presentinvention, hexagonal crystal ferrite powder may be used.

As the hexagonal crystal ferrite, substituent of barium ferrite,strontium ferrite, lead ferrite, calcium ferrite, or Co substituent maybe used. More concretely, magnetoplumbite type barium ferrite andstrontium ferrite, magnetoplumbite type ferrite with particle surfacecovered with spinel, or magnetoplumbite type barium ferrite andstrontium ferrite partially containing spinel phase may be used. It maycontain, in addition to the designated atoms, the following atoms: Al,Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re,Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, Nb, etc.In general, substances added with element such as Co—Ti, Co—Ti—Zr,Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, Nb—Zn, etc. may be used. Thesesubstances containing impurities, which are unavoidably contained due toraw materials and manufacturing methods, may be used.

Particle size is preferably 10 to 200 nm in hexagonal diameter, or morepreferably 20 to 100 nm.

When reproduction is performed on the magnetic resistance head, it isnecessary to decrease noise, and the plate diameter of 40 nm or lower ispreferable. If it is lower than 10 nm, stable magnetization cannot beachieved due to thermal fluctuation. If it is 200 nm or more, noise isincreased. None of these cases is suitable for high-density magneticrecording. Planar ratio (plane diameter/plane thickness) is preferablyin the range of 1 to 15, or more preferably 2 to 7. If planar ratio islower, filling ratio in the magnetic layer is increased and this isdesirable, but orientation property is not good enough. If it is higherthan 15, noise is increased due to stacking between particles. Specificsurface area according to BET method of the particle size range is 10 to200 m²/g. Specific surface area approximately agrees with the valuearithmetically calculated from particle plate diameter and platethickness. Crystallite size is preferably 5 to 45 nm, or more preferably10 to 35 nm. Normally, the narrower the distribution of particle platediameter and plate thickness is, the more it is desirable.Quantification in numerical value is difficult to perform, butcomparison can be made by measuring 500 particles at random on thephotograph taken under transmission electronic microscope (TEM).

Distribution is usually not normal distribution. If it is calculated andexpressed in standard deviation to mean size, it is: σ/mean size=0.1 to2.0. To make the particle size distribution sharper, reaction system togenerate particles is turned to more homogeneous, and distributionimproving processing is carried out on the generated particles. Forinstance, a method is known, in which ultra-fine particles areselectively dissolved in acid solution. Coercive force (Hc) of themagnetic material can be produced to 39.8 to 397.9 kA/m. The higher thevalue of Hc is, the more advantageous it is for high-density recording,but there is limitation due to the ability of the recording head.Normally, it is up to about 63.7 to 318 kA/m, or more preferably 119kA/m to 279 kA/m. In case saturation magnetization of the head exceeds1.4 Tesla, it is preferable to set the value of Hc to 159 kA/m or more.The value of Hc can be controlled by particle size (plate diameter andplate thickness), type and quantity of the elements contained,substitution site of element, and reactive condition to generateparticles. Saturation magnetization (σs) is in the range of 40 to 80Am²/kg. The higher the value of σs is, the more it is desirable. Thefiner the particles are, the more the value of σs decreases. Variousmethods are known to improve the value of σs, i.e. a method to combinespinel ferrite with magnetoplumbite ferrite, or a method to select typeand adding quantity of the elements. Also, W type hexagonal crystalferrite may be used.

When the magnetic material is dispersed, the surface of the magneticparticles may be processed using a dispersion agent or a materialsuitable for polymer. As surface processing material, inorganic compoundor organic compound is used. Typical compounds used are: oxide orhydroxide of Si, Al, P, etc., various types of silane coupling agents,or various types of titanium coupling agents. The adding quantity is 0.1to 10 weight parts to 100 weight parts of the magnetic material.

For the dispersion, pH value of the magnetic material is important. Itis normally about 4 to 12, and optimal value is determined according tothe dispersion agent and polymer. For chemical stability andpreservation property of the medium, pH value of about 6 to 10 isgenerally chosen. Moisture content in the magnetic material also exertsinfluence on dispersion. There is the optimal value depending on thedispersion agent and polymer. Normally, the values of 0.01 to 2.0 weight% is selected.

The following methods are used to produce hexagonal crystal ferrite: (1)Metal oxide to substitute barium oxide, iron oxide and iron are mixedwith boron oxide to use as glass generating substance to obtain ferritecomposition as desired. Then, the mixture is melted and quickly cooledto turn to amorphous substance. Then, it is heated again and is thenwashed and pulverized to barium ferrite crystal powder. This is calledglass crystallization method. (2) Solution of barium ferrite compositionmetal salt is neutralized with alkali. After removing side products, itis heated in liquid phase at 100° C. or more. Then, it is washed, driedand pulverized, and barium ferrite crystal powder is obtained. This iscalled hydrothermal reaction method. (3) Solution of barium ferritecomposition metal salt is neutralized with alkali. After removing sideproducts, it is dried and processed at temperature of lower than 1100°C. Then, it is pulverized and barium ferrite crystal powder is obtained.This is called coprecipitation method. Any of the above methods may beused.

The diamond particles used in the present invention preferably hasaverage particle size in the range of 0.05 to 1.0 μm, or more preferablyin the range of 0.05 to 0.8 μm. If average particle size is less than0.05 μm, the effect to improve durability is decreased. On the otherhand, if it exceeds 1.0 μm, durability is high, but surface property ofthe magnetic layer is decreased, and electromagnetic transfercharacteristics are poor. In the present invention, maximum diameter ofeach diamond particle is defined as particle diameter, and the term“average particle size” used in the present invention is defined as anaverage value of the measured particle diameters in 500 particlesrandomly sampled and photographed under electronic microscope.

Adding quantity of the diamond particles is preferably 0.01 to 5 weight% to the ferromagnetic powder, or more preferably 0.03 to 3.00 weight %.If it is less than 0.01 weight %, it is difficult to maintaindurability. If it is more than 5 weight %, the effect to decrease noiseby addition of diamond particles is lowered. From the viewpoints ofnoise and durability, adding quantity and average particle size of thediamond particles are limited to the above range. From the viewpoint ofnoise, it is preferable that the adding quantity of the diamondparticles is as low as possible. In the magnetic recording medium of thepresent invention, it is preferable that the adding quantity and theaverage particle size of the diamond particles are suitable for magneticrecording and reproducing system adequately selected from the aboverange.

As particle size distribution of the diamond particles, it is preferablethat the number of particles having particles size of more than 200% ofaverage particle size is 5% or less of the total number of diamondparticles, and that the number of particles having particle size of 50%of average particle size accounts for 20% or less of the total number ofthe diamond particles.

To determine particle size distribution, the number of particles iscounted with average particle size as reference when the particle sizeas given above is determined. The particle size distribution of thediamond particles exerts influence on durability and noise. Whenparticle size distribution is more extensive than the above range, theeffects to match average particle size as set in the present inventionmay be changed. Specifically, if there are more particles with largerparticle size, noise may increase or head may be damaged. If there aremore particles with finer particle size, polishing effects are notsatisfactory. The diamond particles with extremely narrow particle sizedistribution are more expensive in cost, and it is more advantageous touse the diamond particles in the above range in terms of cost. Diamondparticles have high hardness. When the diamond particles are used, whichhave sharp particle size distribution and in finer particle size as inthe present invention, the same polishing effects can be obtained withless quantity of abrasives compared with the conventional case, and thisis advantageous from the viewpoint of noise.

Further, abrasives used commonly in the past, e.g. abrasives such asalumina, SiC, etc. may be simultaneously used with the diamond particlesin the present invention. When small quantity of diamond particles onlyis used, effects on durability and S/N ratio are satisfactory. From thereasons such as cost, other abrasives such as alumina, silicon carbide,etc. may be added. In this case also, the diamond particles are added,and adding quantity of alumina can be decreased compared with thequantity necessary to maintain durability, and this is desirable fromthe viewpoints such as maintenance of durability or the decrease ofnoise.

Inorganic powder used in the primer layer of the present invention isnonmagnetic powder. For example, inorganic powder can be selected frominorganic compound such as metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, metal sulfide, etc. As the inorganiccompounds, the following substances are used in combination or alone:For example, α-alumina with alpha ratio of 90% or more, β-alumina,γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide,α-iron oxide, goethite, corundum, silicon nitride, titanium carbide,titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungstenoxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate,calcium sulfate, barium sulfate, molybdenum disulfide, etc. Inparticular, it is preferable to use titanium dioxide, zinc oxide, ironoxide or barium sulfate. More preferably, titanium dioxide or α-ironoxide is used because particle size distribution is lower and there aremany means for providing the functions.

Particle size of the non-magnetic powder is preferably in the range of0.005 to 2 μm. When necessary, non-magnetic powder with differentparticle sizes may be mixed together or similar effect can be providedwith single type of non-magnetic powder by widening the particle sizedistribution. In particular, it is preferable that average particle sizeof the non-magnetic powder is 0.01 to 0.2 μm. In case nonmagnetic powderis particulate metal oxide, average particle size is preferably 0.08 μmor lower. In case it is needle-like metal oxide, it has preferablylonger axis diameter of 0.3 μm or less. Tap density is 0.05 to 2 g/ml,or more preferably 0.2 to 1.5 g/ml. Moisture content of the nonmagneticpowder is preferably 0.1 to 5 weight %, or more preferably 0.2 to 3weight %, or most preferably 0.3 to 1.5 weight %. Also, pH value of thenonmagnetic powder is preferably in the range of 2 to 11, or morepreferably 5.5 to 10. Specific surface area of the nonmagnetic powder ispreferably in the range of 1 to 100 m²/g, or more preferably 5 to 80m²/g, or most preferably 10 to 70 m²/g. Crystallite size of thenonmagnetic powder is preferably in the range of 0.004 to 1 μm, or morepreferably 0.04 to 0.1 μm. Oil absorption using DBP (dibutyl phthalate)is preferably in the range of 5 to 100 ml/100 g, or more preferably 10to 80 ml/100 g, or most preferably 20 to 60 ml/100 g. Specific gravityis preferably in the range of 1 to 12, or more preferably 3 to 6. Theshape of the nonmagnetic powder may be any of needle-like, spherical,polygonal, or planar shape.

Ignition loss is preferably 20 weight % or less. Most preferably, thereis no ignition loss. Morse hardness of the nonmagnetic powder used inthe present invention is preferably 4 or more and 10 or less. Roughnessfactor of the surface of the powder is preferably in the range of 0.8 to1.5, or more preferably 0.9 to 1.2.

Stearic acid adsorption of the nonmagnetic powder is preferably in therange of 1 to 20 μmol/m², or more preferably 2 to 15 μmol/m², or mostpreferably 3 to 8 μmol/m². Heat of wetting of the nonmagnetic powder towater at 25° C. is preferably in the range of 0.2 to 0.6 J/m². Also, asolvent with heat of wetting in the above range can be used. It ispreferable that pH value is in the range of 3 to 6. The nonmagneticpowder preferably contains water-soluble sodium in the range of 0 to 150ppm, and water-soluble calcium in the range of 0 to 50 ppm. It ispreferable that surface of the non-magnetic powder is processed bysurface treatment using Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂O₃, ZnO₂ orY₂O₃. To ensure better dispersion property, it is preferable to useAl₂O₃, SiO₂ or ZrO₂. These substances may be used in combination oralone. According to each individual purpose, coprecipitated surfacetreatment layer may be used, or a method to treat the surface layerusing silica after treating with alumina, or a method reversing thisprocedure may be adopted. The surface treatment layer may be porousaccording to the purpose. In general, it is preferably homogeneous anddense.

Concrete examples of the nonmagnetic powder used in the primer layer ofthe present invention are: Nanotite (manufactured by Showa Denko Co.,Ltd.), HIT-100 and ZA-G1 (manufactured by Sumitomo Chemical IndustryCo., Ltd.), α-hematite DPN-250, DPN-250BX, DPN-245, DPN-270BX, DBN-SA1,and DBN-SA3 (manufactured by Toda Industry Co., Ltd.), titanium oxideTTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, α-hematiteE270, E271, E300, and E303 (manufactured by Ishihara Industry Co.,Ltd.), titanium oxide STT-4D, STT-30D, STT-30, STT-65C, and α-hematiteα-40 (manufactured by Titanium Industry Co., Ltd.), MT-100S, MT-100T,MT-150W, MT-500B, MT-600B, MT-100F, and MT-500HD (manufactured by TeikaCo., Ltd.), FINEX-25, BF-1, BF-10, BF-20, and ST-M (manufactured bySakai Chemical Industry Co., Ltd.), DEFIC-Y and DEFIC-R (manufactured byDowa Mining Co., Ltd.), AS2BM and TiO₂ P25 (manufactured by JapanAerogil), and 100A and 500A and fired products of these materials(manufactured by Ube Industries, Ltd.). The most preferable nonmagneticpowders are titanium dioxide and α-iron oxide.

When carbon black is mixed in the primer layer, it is possible todecrease surface electrical resistance Rs as already known, and lighttransmittance can be decreased. Also, micro-Vickers hardness as desiredcan be obtained. Further, by adding carbon black to the primer layer, itis possible to obtain good effect for storage of lubricant. The types ofcarbon black used for this purpose are: Furnace black for rubber,thermal black for rubber, black for color, acetylene black, etc. In thecarbon black to be added to the primer layer, the following propertiesshould be optimized, and when these are simultaneously used, bettereffects can be attained.

Specific surface area of the carbon black is preferably 100 to 500 m²/g,or more preferably 150 to 400 m²/g. DBP oil absorption is preferably 20to 400 ml/100 g, or more preferably 30 to 200 ml/100 g. Particle size ofthe carbon black is preferably 5 to 80 nm, or more preferably 10 to 50nm, or most preferably 10 to 40 nm. In the carbon black, it ispreferable that pH value is 2 to 10, moisture content is 0.1 to 10%, andtap density is 0.1 to 1 g/ml. Concrete examples of the carbon black tobe used in the present invention are as follows: Blackpearls 2000, 1300,1000, 900, 800, 880, 700, and Vulcan XC-72 (manufactured by Cabot),#3050B, #3150B, #3250B, #3750B, #3950B, #950, #650B, #970B, #850B,MA-600, MA-230, #4000, and #4010 (manufactured by Mitsubishi ChemicalIndustry Co., Ltd.), Conductex SC, Raven 8800, 8000, 7000, 5750, 5250,3500, 2100, 2000, 1800, 1500, 1255, and 1250 (manufactured by ColumbiaCarbon Co.), and Ketchenblack EC (manufactured by Akzo). Carbon blackmay be processed by surface treatment using dispersion agent, or it maybe graphitized with resin and used, or a part of the surface may begraphitized and used. Carbon black may be dispersed with the binder inadvance before it is added to the coating material. These types ofcarbon black can be used within the range not exceeding 50 weight % tothe inorganic powder and within the range not exceeding 40% of totalweight of the primer layer. These types of carbon black may be usedalone or in combination. For further details of the carbon black to beused in the present invention, reference should be made to: “Handbook ofCarbon Black” (compiled by Carbon Black Association of Japan).

Also, organic powder may be added to the primer layer. For example,acrylstyrene type resin powder, benzoguanamine resin powder, melamineresin powder, or phthalocyanine type pigment may be used. Also,polyolefin type resin powder, polyester type resin powder, polyamidetype resin powder, polyimide type resin powder, or polyfluoride ethyleneresin may be used. The methods for manufacturing these types of resinpowder are described in JP-62018564(A) and JP-600255827 (A)

As the binder resin, lubricant, dispersion agent, additive, solvent,dispersing procedure, etc. of the primer layer, those for the magneticlayer as described below can be applied. In particular, for the quantityand type of resin in the binder, adding quantity and type of theadditive and dispersion agent, the technique already known for themagnetic layer can be applied.

The content of the binder in the primer layer is preferably 15 to 40weight parts to 100 weight parts of the nonmagnetic powder, and it ispreferable that the content of the binder is higher in the lower layer.

A coating solution prepared from the above materials is coated on anonmagnetic support member, and a lower nonmagnetic layer is formed. Asthe nonmagnetic support member to be used in the present invention,polyethylene naphthalate, polyethylene terephthalate, polyamide,polyimide, polyamideimide, aromatic polyamide, polybenzoxidazole, etc.processed by biaxial stretching may be used. More preferably,polyethylene naphthalate and aromatic polyamide may be used. Thesenonmagnetic support member may be processed in advance by coronadischarge, plasma treatment, process to make more easily adhesive, heattreatment, etc. Also, the nonmagnetic support member used in the presentinvention preferably has surface with good smoothness, i.e. averagesurface roughness on the central line in the range of 0.1 to 20 nm, ormore preferably 1 to 10 nm with cutoff value of 0.25 mm. Also, it ispreferable that these nonmagnetic support members have not only loweraverage surface roughness on the central line but also have no coarseprojection of higher than 1 μm.

The thickness of the nonmagnetic support member of the magneticrecording medium of the present invention is preferably in the range of4 to 100 μm.

On the surface not coated with the magnetic coating material of thenonmagnetic support member of the present invention, a back-coatinglayer (backing layer) may be provided. The back-coat layer is a layercoated on the surface of the nonmagnetic support member where themagnetic coating material is not coated and where a coating material forforming the back-coat layer is coated. The coating material for formingthe back-coat layer is obtained by dispersing granular components suchas abrasives, anti-static agents, etc. and a binder in organic solvent.As the granular components, various types of inorganic pigments orcarbon black may be used. As the binder, nitrocellulose, phenoxy resin,vinyl chloride type resin, or polyurethane may be used alone or as amixture of these components. An adhesive layer may be provided on thesurface where the magnetic coating material and the coating material forforming the back-coat layer is coated on the nonmagnetic support memberof the present invention.

To produce the magnetic recording medium of the present invention, acoating solution for the primer layer and a magnetic coating solutionare coated to a given thickness on the surface of the nonmagneticsupport member under running condition. The coating solution for theprimer layer and the coating solution for the magnetic layer may becoated sequentially or in multiple layers at the same time. As thecoating device to coat the coating solution for the primer layer or themagnetic coating solution, the following devices may be used: air doctorcoat, blade coat, rod coat, extrusion coat, air knife coat, squeezecoat, impregnation coat, reverse roll coat, transfer roll coat, gravurecoat, kiss coat, cast coat, spray coat, spin coat, etc.

For further details, reference should be made, for example, to “TheNewest Coating Technique” published by Sogo Gijutsu Center, Ltd. (May31, 1983). In case the present invention is applied to a magneticrecording medium, the following can be recommended as examples of thecoating device and method:

(1) Using a coating device such as gravure, roll, blade, extrusion, etc.generally applied in the coating of the coating material, the primerlayer is coated at first. While the primer layer is not yet dried, theupper layer is coated using a support pressurizing type extrusioncoating device as disclosed, for example, in JP-B-88046186,JP-A-60-238179, JP-A-2-265672, etc.

(2) Using a coating head having two slits for allowing the coatingsolution to pass as disclosed in JP-A-63-88080, JP-A-2-17971 orJP-A-2-265762, the upper and the lower layers are coated almost at thesame time.

(3) Using an extrusion coating device equipped with backup roll asdisclosed in JP-A-2-174965, the upper and the lower layers are coatedalmost at the same time. The coating layer of the coating solution forthe magnetic layer is dried after magnetic field orientation processingis performed on the ferromagnetic powder contained in the coating layerof the coating solution for the magnetic layer.

After it has been dried as described above, surface smootheningtreatment is carried out on the coating layer. For the surfacesmoothening treatment, super calender roll is used, for example. By thesurface smoothening treatment, the voids generated due to removal of thesolvent during drying are eliminated, and filling ratio of theferromagnetic powder in the magnetic layer is improved. This makes itpossible to obtain a magnetic recording medium having highelectromagnetic transfer characteristics. As the calender processingroll, heat-resistant plastic roll made of epoxy resin, polyimide,polyamide, polyamideimide, etc. is used. Or, a metal roll may be used.

To perform the processing, the magnetic layer formed by selecting aspecific type of ferromagnetic powder and binder as described above isprocessed by the above calender processing. The conditions for thecalender processing are as follows: The temperature of calender roll ispreferably in the range of 60° C. to 100° C., or more preferably 70° C.to 100° C., or most preferably 80° C. to 100° C. The pressure ispreferably within the range of 98.0 to 490 kN/m, or more preferably 196to 441 kN/m, or most preferably 294 to 392 kN/m. The magnetic recordingmedium thus obtained can be cut to the size as desired using a cutter.

The thickness of the magnetic layer of the magnetic recording medium ofthe present invention is preferably in the range of 0.05 to 1 μm.Because the thickness of the magnetic layer is set to as thin as 0.05 to1 μm in the magnetic recording medium of the present invention, amagnetic recording medium having high electromagnetic transfercharacteristics can be obtained.

On the other hand, in case a thin magnetic layer is provided directly onthe nonmagnetic support member, durability may not be satisfactory evenwhen triester compound with excellent lubricating property may be added.Also, smoothness of the magnetic layer is not satisfactory. Noise ishigh, and high electromagnetic transfer characteristics cannot beobtained. However, when the primer layer is formed as in the presentinvention and the triester compound is added at least to the primerlayer, if the magnetic layer is coated on it and dried and is furtherprocessed by calendering, the surface with superb smoothness havingaverage surface roughness on the central line in the range of 1.0 to 3.5nm, or more preferably 1.0 to 3.0 nm with cutoff value of 0.25 mm can beobtained.

As described above, the magnetic recording medium of the presentinvention is characterized in that a magnetic layer with very smoothsurface can be formed, and it has high durability—in particular, highdurability under high temperature and high speed conditions.

In particular, the combination with the primer layer is important forthe improvement of the surface smoothness, and such smoothness cannot beexpected in the conventional type magnetic recording medium with singlemagnetic layer.

Even when the tetraester compound is added only to the primer layer, itcomes out gradually to the surface of the magnetic layer after themagnetic recording medium has been produced, and the effects to improvethe durability are high.

(Embodiments)

In the following, description will be given on examples of the presentinvention to explain the features of the invention. In the examples, theterm “part(s)” means “weight part(s)”.

EXAMPLES 1 TO 7 AND COMPARATIVE EXAMPLE 3

(Preparation of Magnetic Coating Material A for the Upper Layer)

Ferromagnetic alloy powder 100 parts Composition: Fe 70%, Co 30% (atomratio) Hc: 183 kA/m Specific surface area: 55 m²/g σs: 140 Am²/kgCrystallite size: 155 nm Longer axis length: 0.065 μm Acicular ratio: 5Sintering preventive agent: Aluminum compound (Al/Fe atom ratio 8%) Ycompound (Y/Fe atom ratio 6%) 12 parts Vinyl chloride type copolymer(MR110; Nippon Zeon Co., Ltd.) Sulfonic acid-containing polyurethaneresin 3 parts (UR8200; Toyobo Co., Ltd.) (solid matter) α-alumina (asshown in Table 2) Carbon black 3 parts (#50; Asahi Carbon Co., Ltd.)Compound given in Table 1 Stearic acid 2 parts Methyl ethyl ketone 180parts Cyclohexanone 180 parts (Nonmagnetic coating material 2 for thelower layer) Nonmagnetic powder (Titanium oxide) 80 parts Crystal type:rutile Average primary particle size: 0.035 μm Specific surface area byBET method: 40 m²/g pH: 7 TiO₂ content: 90% or more DEP oil absorption:27 to 38 g/100 g Surface treatment agent: Al₂O₃; 0.8 weight % Carbonblack 20 parts Conductex SC-U (Columbia Carbon Co., Ltd.) Vinyl chloridetype copolymer 12 parts (MR110; Nippon Zeon Co., Ltd.) Sulfonicacid-containing polyurethane resin 5 parts (UR8200; Toyobo Co., Ltd.)(solid matter) Compound given in Table 1 Stearic acid 3 parts Methylethyl ketone/cyclohexanone 250 parts (8/2 in weight ratio)

For each of the above coating materials, the components were kneadedusing a kneader and the mixture was dispersed using a sand mill. To thedispersion solution thus obtained, polyisocyanate was added by 10 partsfor the coating solution on the nonmagnetic layer, and by 10 parts tothe coating solution for the magnetic layer. Further, 40 parts each ofbutyl acetate was added to each solution, and this was filtered using afilter with average bore size of 1 μm. Thus, the coating solutions forforming the nonmagnetic layer and for forming the magnetic layer wereprepared.

The coating solution for the nonmagnetic layer thus prepared was coatedby simultaneous multi-layer coating on a polyethylene terephthalatesupport member of 62 μm in thickness and 0.01 μm in surface roughness onthe central line so that the thickness after drying was to be 1.5 μm andthe thickness of the magnetic layer was to be 0.2 μm immediatelythereafter. While both layers were still in wet condition, this waspassed through an AC magnetic field generating system under theconditions of 50 Hz in frequency and 2.5 mT in magnetic field intensityand of 50 Hz in frequency and 1.2 mT in magnetic field intensityrespectively to perform random orientation processing. After drying,this was processed by 7-stage calendering under the condition of 90° C.in temperature and 294 kN/m in linear pressure. This was punched toprepare a 3.7-type magnetic recording medium. After surface polishing,this was placed in a 3.7-type cartridge (for Zip Drive; Iomega Inc.)with a liner provided on inner side, and a 3.7-type floppy disk wasprepared.

Each specimen of the floppy disks thus produced was measured by theevaluation methods given below.

(Evaluation Methods)

1. Reproduction Output

Measurement of S/N ratio

Using a disk evaluation system (RAW 1001; Guzik Inc., U.S.A.), a spinstand (LS-90; Kyodo Electronics System, Co., Ltd.), and a metal-in gaphead of 0.3 μm in gap length, the specimen was placed at a position of24.6 mm in radius. Reproduction output (TAA) at linear recording densityof 90 kfci and noise after DC demagnetization were measured, and SINratio was obtained.

2. Durability

Using a floppy disk drive (ZIP 100, Iomega, Inc.; number of revolutions2968 rpm), the head was fixed at a position of 38 mm in radius.Recording was performed at recording density of 34 kfci, and the signalwas reproduced, and this was regarded as 100%. Then, it was operated for1,000 hours under thermo-cycle environment with the course of flow givenbelow as one cycle. At every 24 hours of running operation, the outputwas measured. When the output was turned to 70% or less of the initialvalue, it was defined that the service life has expired.

Thermo-cycle Flow Test:

Then, the procedure was returned to A, and the cycle of A→B→C→D→A wasrepeated.

TABLE 1 Compound structure R¹ R², R³, R⁴ Compound A General formula (1)—C₂H₅ —CH₂(CH₂)₃CH₃ Compound B General formula (1) —C₂H₅ —CH(CH₂CH₃)(CH₂)₃CH₃ Compound C General formula (1) —C₂H₅ —CH₂(CH₂)₅CH₃ Compound DGeneral formula (1) —C₂H₅ —CH₂(CH₂)₉CH₃ Compound E General formula (1)—CH₃ —CH₂(CH₂)₁₃CH₃ Compound F General formula (2) — —CH₂(CH₂)₉CH₃Compound G n-butylstearate

TABLE 2 Compound Diamond Non- Average α-alumina Upper magnetic particleAdding Adding Adding Dura- magnetic lower size quantity quantityquantity S/N bility layer layer (μm) (parts) (μm) (parts) (dB) (hr)Example 1 A A 0.25 3 — — 5.2 1600 Example 2 B B 0.25 3 — — 5 1700Example 3 C C 0.25 3 — — 5.6 1600 Example 4 D D 0.25 3 — — 6.2 1500Example 5 E E 0.25 3 — — 5.8 1500 Example 6 F F 0.25 3 — — 5.4 1500Example 7 G A 0.25 3 — — 5 1500 Example 8 A A 0.05 3 — — 5.6 1500Example 9 A A 1 3 — — 5 1500 Example 10 A A 0.25 3 0.25 10 4 1700Comparative A A — — 0.25 10 4.2  400 example 1 Comparative A A — — 0.2520 2  700 example 2 Comparative A A — — 0.25 40 0 1300 example 3Comparative G G 0.25 3 — — 4  300 example 4 Comparative G G — — 0.25 203  200 example 5 Comparative A A — — 0.05 20 2.4  400 example 6Comparative A A — — 1 20 1.6  700 example 7

As described above, according to the present invention, at least aspecific type of an ester compound is used as a lubricant for the primerlayer and diamond particles are added to the magnetic layer. As aresult, electromagnetic transfer characteristics on the high-densityrecording medium can be improved, and durability can be increased on ahigh recording density disk type medium.

What is claimed is:
 1. A magnetic recording medium, comprising anonmagnetic primer layer placed on a nonmagnetic support member and atleast one layer of magnetic layers with ferromagnetic powder dispersedin a binder, said magnetic layer being placed on said primer layer,whereby diamond particles with average particle size of 0.05 to 1 μm areadded to the magnetic layer, and at least one type of compound selectedfrom the following genera formulae (1) and (2) is contained:

where R¹ represents an alkyl group having 1 to 2 carbon atoms; and R² ,R³ and R⁴ each represents a hydrocarbon group having 4 to 21 carbonatoms.
 2. A magnetic recording medium according to claim 1, whereinthickness of said magnetic layer is in the range of 0.05 to 1 μm.
 3. Amagnetic recording medium according to one of claims 1 or 2, wherein themedium is a disk type magnetic recording medium.